Technical Field
The present invention relates to a high-resolution digital microphone having wide dynamic range and bandwidth, and a position-to-frequency converter that can serve as a position sensor.
Background Art
Typical condenser microphones include conductive membranes or diaphragms that vibrate in response to the incident acoustic waves, and electrodes fixed so as to face the membranes. Such a membrane and fixed electrodes constitute a capacitor. The condenser microphone includes the capacitor and a buffer amplifier or a preamplifier that reads the variation in capacitance of the capacitor caused by a shift in the membrane.
The recent digitization of acoustic devices often requires the output signals of microphones to be digital signals. In the condenser microphones, analog signals output from the buffer amplifiers or preamplifiers are converted into digital signals with analog-to-digital converters (ADCs). Such a digitizing scheme is a well-known technique and is put to wide practical use.
With reference to FIG. 4, a conventional digital microphone includes an ADC. The main component of the ADC is a delta-sigma (ΔΣ) modulator 84. Analog signals 100 or audio signals electro-acoustically converted in a microphone unit (e.g., condenser microphone unit) 80 are sent to the A modulator 84 via a preamplifier 82. The signals output from the ΔΣ modulator 84 pass through a digital filter 86. The ΔΣ modulator 84 operates in synchronization with high-frequency clock signals 102 and converts the analog signals 100 into digital signals 104 modulated by single-bit pulse-density. The ΔΣ modulator 84 includes an integrator, a single-bit quantizer, and a digital-to-analog converter. This configuration is well known, and thus, details of the components will be omitted.
The ΔΣ modulator 84 oversamples the analog signals 100, i.e., samples the analog signals 100 with a frequency significantly higher than the audio signal band, and converts the analog signals into the digital signals 104 modulated by single-bit pulse-density. This process achieves noise-shaping effect, that is, the quantized noise is shifted toward the high frequency band.
PTL 1 describes a digital microphone including a ΔΣ modulator. The digital microphone according to PTL 1 is essentially the same as a typical digital microphone including an ADC. The performance of a digital microphone including an ADC depends on the performance of the ADC, and the performance of the ADC depends on the performance of an analog circuit. A difficult task is designing analog circuits, especially analog circuits having wide bands and wide dynamic ranges for professional use and ultrasonic sensing.
The inventors have proposed a digital microphone that can directly output digital audio signals from the microphone unit and be satisfactorily put to professional use (refer to PTL 2). With reference to FIG. 5 illustrating the digital microphone, a condenser microphone serves as a capacitor that establishes the oscillatory frequency of an oscillator to convert the shift of a membrane in response to incident acoustic waves into frequency.
With reference to FIG. 5, a resonator 12 disposed on a substrate 14 faces a conductive membrane 10. The substrate 14 is secured to a back plate 16. The resonator 12 on the substrate 14 has a wiring pattern on the surface facing the membrane 10. The vibration of the membrane 10 varies the capacitance between the membrane 10 and the resonator 12. The variation in this capacitance causes a variation in the resonance frequency of the resonator 12. The resonator 12 is included in an oscillator that outputs FM signals 110 in response to the vibration of the membrane 10.
The FM signals 110 are sent to a single-bit quantizer 18. The single-bit quantizer 18 samples the FM signals 110 with a high-frequency clock signals 102 and outputs single-bit quantized signals. The single-bit quantized signals are sent to one of the input terminals of an exclusive OR (XOR) circuit 22 directly and to the other input terminal via a resistor 20. The resistor 20 operates in synchronization with the high-frequency clock signals 102 each having a frequency that is the same as the sampling frequency and sends the delayed signals after single-bit quantization to the XOR circuit 22.
The resistor 20 and the XOR circuit 22 constitute an edge detector that detects the edges of the single-bit quantized signals from the single-bit quantizer 18. The output from the XOR circuit 22 is in the form of a ΔΣ-modulated signal 112. The single bit output is reduced to the Nyquist rate through a digital filter so as to acquire a digital signal having high resolution and a wide dynamic range.
A conventional digital microphone including a ΔΣ modulator has the advantages described above but also has issues that should be solved. One of such issues is the ready decrease in the Q value of the resonator. The noise floor of a ΔΣ modulation scheme, i.e., the noise level without signal input is determined by the phase noise of the oscillator. Thus, the resonator requires a large Q value. A resonator including a capacitor and an inductor, such as the resonator according to the related art, has a Q value of at most 100, usually in the range of 10 to 20. A relaxation oscillator including a variable capacitor has a smaller Q value and generates larger phase noise.
A second issue concerning a conventional digital microphone including a ΔΣ modulator is the low rate of variation of frequency in response to the shift of a membrane. The membrane of the microphone is fixed at the periphery. Thus, the shift of the peripheral area of the membrane is small compared to the shift of the central area. As a result, a variation in the electrostatic capacitance is small relative to a large shift in the central area. For an LC resonator, the resonance frequency depends on the square root of the product of inductance L and capacitance C. Thus, the variation in frequency is even smaller. The SN ratio and the dynamic range of an ADC including a ΔΣ modulator strongly depend on the frequency modulation width. Thus, the small variation in frequency interferes with the enhancement of the SN ratio and the dynamic range.